Question

What happens when ${{P}_{4}}$ and $A{{s}_{4}}$ are treated separately with hot and concentrated nitric acid?

Answer 1

Most of the times, when an element is subjected to a reaction with hot and concentrated nitric acid, the acid of that element is formed as a product. Nitrogen dioxide and water are obtained as by-products in respective reactions.

Complete answer:
The two elements given in the question are phosphorus ( ${{P}_{4}}$ ) and arsenic ( $A{{s}_{4}}$ ), when they are subjected to reaction with hot and concentrated acid separately their respective acids are formed.
In case of phosphorus, phosphoric acid will be formed, and nitrogen dioxide and water are formed as by-products.
The reaction for the formation of phosphoric acid is given below.
${{P}_{4}}+20HN{{O}_{3}}\xrightarrow{{}}4{{H}_{3}}P{{O}_{4}}+20N{{O}_{2}}+4{{H}_{2}}O$
Here, 4 atoms of phosphorus react with 20 molecules of nitric acid, to give 4 molecules of phosphoric acid, 20 molecules of nitrogen dioxide and 4 molecules of water.
In phosphoric acid, phosphorus contributes positive 5 oxidation number, thus to balance the equation 20 molecules of nitric acid react with 4 atoms of phosphorus to give 4 molecules of phosphoric acid,
In case of arsenic, arsenic acid will be formed, and again nitrogen dioxide and water will be formed as by-products.
The reaction for the formation of arsenic acid is given below.
$A{{s}_{4}}+20HN{{O}_{3}}\xrightarrow{{}}4{{H}_{3}}As{{O}_{4}}+20N{{O}_{2}}+4{{H}_{2}}O$
Here also, 4 atoms of arsenic react with 20 molecules of nitric acid to give 4 molecules of arsenic acid. Also, the oxidation number of arsenic is positive 5 and thus 20 molecules of nitric acid react with 4 atoms of arsenic to give 4 molecules of arsenic acid.

Note:
It is not always necessary that any element that will react with hot and concentrated nitric acid will form its own acid, antimony forms antimony oxide and not antimony acid. Also, when sulphur reacts with hot and concentrated nitric acid, it gives sulphuric acid.